Chitin and chitosan have been attracting interests as naturally-occurring functional polysaccharides, and have found utility in a variety of fields. Chitin and chitosan are common names, and there is no officially-unified definition that clearly distinguishes them from each other. In the present invention, a polysaccharide composed solely of N-acetylglucosamine will be called “chitin”, and a polysaccharide composed of N-acetylglucosamine and glucosamine in combination or composed solely of glucosamine will be called “chitosan”. Chitosan is known to be found in nature, and is also one of a few aminopolysaccharides which are produced industrially. Functions originated from its amino groups, for example, its absorptive ability for acidic substances and the resulting cationic property are utilized in a multiplicity of fields. Specifically, those obtained by combining chitosan and fibers together by various methods are marketed as antimicrobial anti-odor fibers.
One of most convenient methods for combining fibers and chitosan together is to coat or impregnate fibers or a fibrous product with a slightly acidic aqueous solution of chitosan and then to dry the coated or impregnated fibers or fibrous product. Through treatment by this method, an antimicrobial anti-odor function can be imparted to the fiber product. According to this treatment, however, the chitosan applied to the fibers has not been water-insolubilized, so that the chitosan is readily eliminated from the fibers by washing and no washing fastness can be assured for so-called chitosan-treated fibers. Not only for the assurance of washing fastness for chitosan but also for the utilization of chitosan, it is hence an important key technology to water-insolubilize chitosan as desired. A variety of methods have, therefore, been attempted with a view to water-insolubilizing chitosan.
For example, it has been attempted to insolubilize chitosan by coating or impregnating a suitable base material with an aqueous solution, in which chitosan is dissolved as a salt (chitosan salt) of chitosan and an acid as a result of addition of chitosan to an acidic aqueous solution, drying the thus-coated or impregnated base material, and then neutralizing the acid with an aqueous solution of an alkali such as caustic soda or sodium bicarbonate to liberate water-insoluble chitosan or by treating a chitosan salt, with which a base material has been coated or impregnated, with a solution of an anionic polymer such as alginic acid or carboxymethylcellulose to form a polyionic complex of chitosan and the anionic polymer. By these methods, a chitosan salt can be water-insolubilized. However, a coating formed of the resulting chitosan is not sufficient in strength, and moreover, the treatment is not a one-solution treatment but a two-solution treatment. The above-described treatment process becomes long. Accordingly, these methods are also disadvantageous from the standpoint of treatment cost. With respect to the water insolubilization of chitosan with crosslinking agents, various methods have also been attempted.
It is to be noted that the term “one-solution” or “one-pack” as used herein means to form a coating with only one kind of chitosan solution while the term “two-solution” as used herein means to form a chitosan coating by using a chitosan solution and another solution, which insolubilizes chitosan (the above-described alkali solution, anionic polymer solution, crosslinking agent solution, or the like), upon formation of the coating.
For example, Patent Document 1 discloses a technology that water-insolubilizes amino-containing chitosan by crosslinking, it with a polyethylene glycol derivative containing glycidyl ether groups, and Patent Document 2 describes a technology that water-insolubilizes water-soluble chitosan by crosslinking it with a water-soluble urethane prepolymer containing carbamoyl sulfonate groups at molecule ends. These technologies both require heat treatment at 150° C. or so upon formation of a coating, and therefore, cannot be used in applications in which no high-temperature treatment can be conducted, such as treatment of heat-sensitive paper.
On the other hand, a mixed solution prepared by mixing a dialdehyde compound such as glyoxal or glutaraldehyde in a chitosan salt solution can obtain a water-insoluble coating even by room-temperature drying, to say nothing of heated drying, after coating or impregnation of a base material. Although the mixed solution is in the form of a “single pack”, this “single-pack” solution has a short pot life (useful lifetime). A limitation is thus imposed on its use. More importantly, crosslinked chitosan coatings formed using these dialdehydes unavoidably undergo severe yellowing with time. There is, accordingly, an outstanding desire for the development of a “single-pack” type chitosan solution that can form a crosslinked chitosan coating with reduced yellowing tendency.
As one of methods for insolubilizing a chitosan coating obtainable from a chitosan solution which is a “single pack” form and is provided with improved pot life performance, the present inventors disclosed that an aqueous solution of chitosan and a polybasic acid remains stable as a “one-pack” solution at room temperature over a long term and also that, when subjected to heat treatment at 150° C. or higher after once a base material is coated or impregnated with the aqueous solution and then dried, the resulting chitosan coating becomes water-insoluble (Patent Document 3). This method can obtain a chitosan coating having excellent waterproofness by heating treatment, but in room-temperature (low-temperature) drying, no chitosan coating can be obtained with excellent waterproofness by this method.
As described above, there has been a desire for the development of a chitosan composition which has a pot life in a “one-pack” form, can water-insolubilize a chitosan coating even by room-temperature drying after coating or impregnation of a base material, and can form a chitosan coating having reduced yellowing tendency.
Patent Document 1: JP-A-11-247067
Patent Document 2: JP-A-4-253705
Patent Document 3: JP-A-2003-206409